Some might balk at the concept of The Devil being a somewhat good guy but if you consider Adam Sandler's religious/ethnic background (Judaism), it makes perfect sense and doesn't come off as "heretical" at all. In Judaism, Satan isn't the embodiment of pure evil and enemy of God but an agent of God who tests humans and punishes sinners. Sandler basically takes elements of the Christian understanding of Hell & the Devil (or rather the Word of DanteTheme Park Version) but makes Satan morally like the Jewish version. Sandler's Satan is less akin to the Zoroastrian Angra Mainyu/Ahriman and is more like the Hindu Deva, Yama mixed with a Trickster Deity.
At first it seems like plot convenience that Nicky masters "Release the Good" much faster than "Release the Evil". But of course Nicky would be skilled in goodness and mediocre at evil, he's a very good person and has lots of practice with being nice.
New York wholeheartedly embraced sin, as Adrian put it with "minimal prompting" indicating that a good chunk of New York was hell-worthy in the first place, also, unless they cover it up somehow, Earth has implicit evidence Hell exists.
